Searching for Earth 2.0? Zoom In On An Star (Planetary Science)

Astronomers searching for Earth-like planets in other solar systems have made a breakthrough by taking a closer look at the surface of stars.

A new technique developed by an international team of researchers — led by Yale astronomers Rachael Roettenbacher, Sam Cabot, and Debra Fischer — uses a combination of data from ground-based and orbiting telescopes to distinguish between light signals coming from stars and signals coming from planets orbiting those stars.

A study detailing the discovery has been accepted by The Astronomical Journal.

“Our techniques pull together three different types of contemporaneous observations to focus on understanding the star and what its surface looks like,” said Roettenbacher, a 51 Pegasi b postdoctoral fellow at Yale and lead author of the paper. “From one of the data sets, we create a map of the surface that allows us to reveal more detail in the radial velocity data as we search for signals from small planets.

“This procedure shows the value of obtaining multiple types of observation at once.”

For decades, astronomers have used a method called radial velocity as one way to look for exoplanets in other solar systems. Radial velocity refers to the motion of a star along an observer’s sightline.

Astronomers look for variations in a star’s velocity that might be caused by the gravitational pull of an orbiting planet. This data comes via spectrometers — instruments that look at light being emitted by a star and stretch the light into a spectrum of frequencies that can be analyzed.

As astronomers have rushed to develop methods for detecting Earth-like planets, however, they have run into a barrier that has stopped progress for years. The energy emitted by stars creates a boiling cauldron of convecting plasma that distorts measurements of radial velocity, obscuring signals from small, rocky planets.

But a new generation of advanced instruments is attacking this problem. These instruments include the EXtreme PREcision Spectrograph (EXPRES), which was designed and built by Fischer’s team at Yale, the Transiting Exoplanet Survey Satellite (TESS), and the Center for High Angular Resolution Astronomy (CHARA) interferometric telescope array.

For the new study, the researchers used TESS data to reconstruct the surface of Epsilon Eridani, a star in the southern constellation of Eridanus that is visible from most of Earth’s surface. They then looked for starspots — cooler regions on the surface of a star caused by strong magnetic fields.

“With the reconstructions, you know the locations and sizes of spots on the star, and you also know how quickly the star rotates,” said Cabot. “We developed a method that then tells you what kind of signal you would see with a spectrometer.”

The researchers then compared their TESS reconstructions with EXPRES spectrometer data collected simultaneously from Epsilon Eridani.

“This allowed us to directly tie contributions of the radial velocity signature to specific features on the surface,” Fischer said. “The radial velocities from the starspots match up beautifully with the data from EXPRES.”

The researchers also used another technique, called interferometry, to detect a starspot on Epsilon Eridani — the first interferometric detection of a starspot on a star similar to the Sun.

Interferometry combines separated telescopes to create a much larger telescope. For this, the researchers used the CHARA Array, the world’s largest optical interferometer, located in California.

Roettenbacher said she and her colleagues will apply their new technique to sets of interferometric observations in order to directly image the entire surface of a star and determine its radial velocity contribution.

“Interferometric imaging is not something that is done for a lot of stars because the star needs to be nearby and bright. There are a handful of other stars on which we can also apply our pioneering approach,” Roettenbacher said.

Former Yale researchers Lily Zhao, who is now at the Flatiron Institute, and John Brewer, who is now at San Francisco State University, are among the study’s co-authors.

The research was supported by the Heising-Simons Foundation, an anonymous Yale alumnus, the National Science Foundation, and NASA.

Featured image: Reconstructed surface of the spotted star Epsilon Eridani with each panel showing the star advanced one-fifth of its rotation. (Visualization by Sam Cabot)

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Provided by Yale University

What Are The Properties of Thorne-Żytkow Object Explosions? (Planetary Science)

Takashi Moriya and Sergei Blinnikov investigated the outcomes of the potential TŻO explosions and shed light on their expected observational properties. Their study recently appeared in Arxiv.

Thorne-Żytkow objects are stars that have a neutron star core with an extended hydrogen-rich envelope. Previously, Takashi Moriya et al. suggested that the massive Thorne-Żytkow objects are proposed to explode when the nuclear reactions sustaining their structure are terminated by the exhaustion of the seed elements.

“We investigated the observational properties of the possible Thorne-Żytkow object explosions.”, told M. Takashi, Researcher at National Astronomical Observatory of Japan.

They showed that, Thorne-Żytkow object explosions are observed as long-duration transients lasting for several years.

They also showed that, if the accretion disk triggering the explosions does not last for a long time, Thorne-Żytkow object explosions have a luminosity plateau with about 10³⁹ erg s¯¹ lasting for a few years, and then they suddenly become faint. They would be observed as vanished stars after a bright phase lasting for a few years.

Additionally, if the accretion disk is sustained for long time, the Thorne-Żytkow object explosions become as bright as supernovae. They would be observed as supernovae with rise times of several hundred days.

Finally, it has been found that their photospheric velocities are 2000 km s¯¹ at most, much smaller than those found in supernovae. Supernovae with extremely long rise times such as HSC16aayt and SN 2008iy may be related to the explosions of Thorne-Żytkow objects.

“While we only have a few SNe that have long-duration light curves similar to our synthetic light curves from TŻO explosions, many long-duration SNe will be found in the coming era of the Rubin Observatory’s Legacy Survey of Space and Time (LSST). Since the faintest TŻO explosions are expected to have luminosities that are similar to massive stars, they can be eventually found as disappeared stars.”, they concluded.

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Reference: Takashi J. Moriya and Sergei I. Blinnikov, “Properties of Thorne-Żytkow object explosions”, Arxiv, 2021. https://arxiv.org/abs/2109.01250

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Astronomers Discovered Super Neptune Around M-dwarf Star (Planetary Science)

Using precision radial velocity (RVs) from the near infrared (NIR) Habitable-zone Planet Finder spectrograph (HPF), a team of international astronomers reported the discovery and confirmation of a super Neptune, TOI-532b, orbiting an M-dwarf star TOI-532 in a ∼ 2.3 day circular orbit. Their findings recently appeared in Arxiv.

TOI-532 is an early type metal-rich M dwarf star located in the constellation of Orion. It was observed by TESS in Sector 6 in Camera 1 from 11 December 2018 to 7th January 2019 at two minute cadence.

Figure 1. Short cadence (2 minute) time series TESS PDCSAP photometry (without detrending) from Sector 6, with the binned data (in 1 hour bins), along with the TOI-532b transits overlaid in blue. © Shubham Kanodia et al.

Now, a team of international astronomers led by Shubham Kanodia reported the discovery of a super neptune orbiting TOI-532. They performed a comprehensive characterization of the stellar and planetary properties using space-based photometric observations from TESS, additional ground-based transit observations, adaptive optics imaging, and high-contrast speckle imaging.

Table 1: Derived Parameters for the TOI-532 System © Shubham Kanodia et al.

They showed that, TOI-532 is a metal-rich M dwarf with an effective temperature of 3957 K and [Fe/H] = 0.38; it hosts a transiting gaseous planet with a period of ∼ 2.3 days. Joint fitting of the radial velocities with the TESS and ground-based transits revealed a planet with radius of 5.82 R, and a mass of 61.5 M (more parameters are shown in Table 1 above).

“TOI-532b is the largest and most massive super Neptune detected around an M dwarf with both mass and radius measurements, and it bridges the gap between the Neptune-sized planets and the heavier Jovian planets known to orbit M dwarfs.”

— they said.

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Additionally, it has been shown that the planet is situated at the edge of the Neptune desert in the Radius–Insolation plane, which will help place constraints on the mechanisms responsible for sculpting this region of planetary parameter space.

Figure 2. They showed TOI-532b (circled) in different planet parameter space along with other M dwarf planets with mass measurements at > 3σ © Shubham Kanodia et al.

Finally, they suggested that, TOI-532 is relatively faint (J = 11.46), but is still accessible from 10-m class telescopes, as a potential target for detecting atmospheric escape using the He 10830 Å triplet.

“The discovery and mass measurement of gas giants such as TOI-532b adds to the small sample of such planets around M dwarf host stars, and can help inform theories of planetary formation and evolution. Therefore, we encourage future observations to place limits on atmospheric escape using the He 10830 Å transition.”

— they concluded.

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Featured image: Neptune Illustration © Getty images

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Reference: Shubham Kanodia, Gudmundur Stefansson, Caleb I. Canas, Marissa Maney, Andrea S. Lin, Joe P. Ninan, Sinclaire Jones, Andrew J. Monson, Brock A. Parker, Henry A. Kobulnicky, Jason Rothenberg, Corey Beard, Jack Lubin, Paul Robertson, Arvind F. Gupta, Suvrath Mahadevan, William D. Cochran, Chad F. Bender, Scott A. Diddams, Connor Fredrick, Samuel P. Halverson, Suzanne L. Hawley, Fred R. Hearty, Leslie Hebb, Ravi K. Kopparapu, Andrew J. Metcalf, Lawrence W. Ramsey, Arpita Roy, Christian Schwab, Maria Schutte, Ryan C. Terrien, John P. Wisniewski, Jason T. Wright, “TOI-532b: The Habitable-zone Planet Finder confirms a Large Super Neptune in the Neptune Desert orbiting a metal-rich M dwarf host”, Arxiv, 2021. https://arxiv.org/abs/2107.13670

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Astronomers Discovered A Warm Sub-Neptune Transiting A Bright Solar Twin (Planetary Science)

Using Transiting Exoplanet Survey Satellite (TESS), a team of international astronomers reported the discovery of a transiting warm sub-Neptune planet, ‘HD 183579b’, around the nearby bright (𝑉 = 8.75 mag, 𝐾 = 7.15 mag) solar twin HD 183579. Their study recently appeared in Arxiv.

HD 183579 is a G2V star with a spectrum nearly identical to that of the Sun. The star has been studied extensively through a dedicated RV planet search and spectroscopic abundance survey targeting solar twin stars with the High Accuracy Radial velocity Planet Searcher spectrograph (HARPS). The transiting planet, however, was not detected until TESS data became available.

Now, a team of international astronomers characterized the HD 183579 planetary system using both space and ground-based photometric data from TESS and LCO as well as the spectroscopic data from HARPS and Minerva-Australis.

They found that the host star is located 56.8 pc away with a radius of 𝑅∗ = 0.97 𝑅 and a mass of 𝑀∗ = 1.03 𝑀. They also found that HD 183579b (TOI-1055b) has a radius of 𝑅𝑝 = 3.53 𝑅 on a 17.47 day orbit with a mass of 𝑀𝑝 = 11.2 𝑀 (3𝜎 mass upper limit of 27.4 𝑀).

Taken together, they found the resulting planetary bulk density of 1.4 g cm¯³, which implies the presence of an extended atmosphere, making this system an excellent candidate for transmission spectroscopic follow-up.

Finally, by performing a line-by-line differential analysis using the high resolution and signal-to-noise ratio HARPS spectra, they found that HD 183579 does not show a similar depletion in the abundance of refractory elements as our Sun.

“HD 183579b is the fifth brightest known sub-Neptune planet system in the sky, making it an excellent target for future studies of the interior structure and atmospheric properties.”

— they concluded.

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Featured image: The mass radius diagram in Earth units. HD 183579b is marked as a red square. The confirmed planets with well measured radius and mass are shown as black points (uncertainty smaller than 20%) while grey points represent the planet with poor constraint (data are retrieved from NASA Exoplanet Archive). The colored lines are the theoretical M-R models for different planetary compositions © Tianjun Gan et al.

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Reference: Tianjun Gan, Megan Bedell, Sharon Xuesong Wang, Daniel Foreman-Mackey, Jorge Meléndez, Shude Mao, Keivan G. Stassun, Steve B. Howell, Carl Ziegler, Robert A. Wittenmyer, Coel Hellier, Karen A. Collins, Avi Shporer, George R. Ricker, Roland Vanderspek, David W. Latham, Sara Seager, Joshua N. Winn, Jon M. Jenkins, Brett C. Addison, Sarah Ballard, Thomas Barclay, Jacob L. Bean, Brendan P. Bowler, César Briceño, Ian J. M. Crossfield, Jason Dittman, Jonathan Horner, Eric L. N. Jensen, Stephen R. Kane, John Kielkopf, Laura Kreidberg, Nicholas Law, Andrew W. Mann, Matthew W. Mengel, Edward H. Morgan, Jack Okumura, Hugh P. Osborn, Martin Paegert, Peter Plavchan, Richard P. Schwarz, Bernie Shiao, Jeffrey C. Smith, Lorenzo Spina, C. G. Tinney, Guillermo Torres, Joseph D. Twicken, Michael Vezie, Gavin Wang, Duncan J. Wright, Hui Zhang, “HD 183579b: A Warm Sub-Neptune Transiting a Solar Twin Detected by TESS”, Arxiv, pp. 1-20, 2021. https://arxiv.org/abs/2107.14015

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Astronomers Discovered First Heavy Metal Hot Subdwarf Star In SB 744 (Planetary Science)

A team of international astronomers reported the discovery of first heavy-metal hot subdwarf composite binary SB 744. Using radial velocity follow-up they revealed that, the hot subdwarf star in SB 744 have a substantial overabundances of lead and fluorine in its atmosphere. Their study recently appeared in Arxiv.

Hot subdwarf stars are 0.5 M core helium burning stars on the extreme horizontal branch (EHB). Unlike normal horizontal branch stars they retain only a very thin hydrogen envelope after their evolution on the red giant branch (RGB). The envelope is lost under unclear circumstances, precisely by the time they ignite helium burning in the core. The precise timing and high binary fraction hints a yet unknown connection between mass loss and binary evolution.

The spectral properties of hot subdwarfs show a nearly continuous sequence from 25000 K B-type subdwarfs (sdB) to the hottest sdO type stars exceeding 40000 K. However, there are parallel sequences well separated by helium abundance, and, along these sequences, multiple groups can be identified. This granulation in parameter space correlates with other properties, such as binarity and most likely indicate different formation pathways that lead to different sub-classes of hot subdwarfs. The spectra of the intermediate helium-rich sub-class show a mix of He i and ii lines (sdOB type) and their location in the parameter space differ from the canonical picture of sdB and sdO stars. A relation between sdB and sdO stars can be outlined by stellar evolution, where sdB stars are the progeny of post-EHB sdO stars. The similar binary properties of sdB and sdO types confirm this evolutionary link. However, the sdOB subclass shows a very low binary fraction, implying a different formation history. This hypothesis got further support from the studies which demonstrated that heavy-metal over-abundances in several sdOB stars. The trans-iron metal abundances reach over 10 000 times the solar values in these stars. Such heavy metals are produced by the s-process during intermediate evolution within the He-shell burning environments of low metallicity stars. However, whether these sdOB stars have extra amounts of heavy-metals or the observed abundances are the result of diffusion, which places metals selectively into a thin photospheric layer, is not yet clear.

SB 744 (also MCT 0146-2651) is a bright (V=12.31 mag) hot subdwarf composite spectrum binary. It was first identified
by Slettebak & Brundage as the 744th object in their catalog of early type stars near the south Galactic Pole. It is a relatively well studied object, its first quantitative spectral analyses date back to the 1980s. However, the binary nature of the system is still in question.

Fig 1: The orbital period versus the eccentricity and mass ratio for all known composite sdB binaries with solved orbits. The main group is shown in blue filled circles, the secondary group in orange open circles, and SB 744 is highlighted as a red square. The orbital parameters of SB 744 fit with those of the main group. © Nemeth et al.

Now, a team of international astronomers confirmed that SB 744 is a typical composite binary with orbital parameters similar to those of other composite subdwarf binaries. Later, with the help of stellar evolution models, they determined the mass of hot subdwarf and its cool companion which was found to be 0.47 M and 0.72 M, respectively.

In addition, they analyzed the optical spectra with homogeneous atmospheric models to derive surface parameters of the binary members from a direct wavelength space decomposition and independently measured the atmospheric properties of the cool companion.

“While lead has been observed in a dozen of sdOB stars, SB 744 is a first such star that shows a measurable Fluorine abundance”

— they said

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They found that, it is an old, population II system, that has gone through dramatic events. The hot subdwarf star in SB 744 belongs to the heavy-metal subclass of sdOB stars and have a substantial overabundances of lead and fluorine in its atmosphere. But, it also has the iron and helium abundance lowest among the known heavy-metal sdOB stars.

“The companion does not show extra overabundances, and it seems like a normal low metallicity MS star.”

— they said.

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The presence of fluorine implies that SB 744 was once a hierarchical triple system and the inner binary has merged in the near past. As an alternative scenario, single-star evolution through late core helium flash and atmospheric mixing can also produce the observed fluorine abundances. The atmospheric metal over-abundances currently observed are perhaps the results of a combination of mixing processes during formation and radiative support.

“The mass of the sdOB is consistent with the canonical mass and is likely formed from a low-mass progenitor, or a merger.”

— they said.

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Moreover, by using the GALPY software package they calculated the Galactic orbit of SB 744 and found that it is a Halo object, like all other heavy-metal hot subdwarfs discovered to date.

Fig 2: The Galactic orbit of SB 744 projected onto the Galactic plane. The trajectory over a time of 2 Gyr and the current position (red dot) is shown. © Németh et al.

Finally, SB 744 demonstrated that heavy-metal overabundances occur in metal-poor stars, where efficient atomic diffusion can support the Fluorine and Lead abundance in thin photospheric layers.

“The period of atmospheric overabundances are probably temporary, short lived phases compared to stellar evolution. This explains why not all similar stars show heavy-metal over-abundances, which are most likely in connection with mixing events following a He-shell subflash.”

— they added.

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“This study will take the investigations of heavy-metal subdwarfs to a new level and implies that more such subdwarfs may be hidden in composite spectrum binaries.”, they concluded.

Featured image credit: Getty Images

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Reference: P. Németh, J. Vos, F. Molina, and A. Bastian, “The first heavy-metal hot subdwarf composite binary SB 744”, will appear in Astronomy and Astrophysics, preprint:
arXiv:2107.03270

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A Star in a Distant Galaxy Blew Up in A Powerful Explosion, Solving An Astronomical Mystery (Planetary Science)

Giant explosion in space illuminates thousand-year mystery

Dr. Iair Arcavi, a Tel Aviv University researcher at the Raymond and Beverly Sackler Faculty of Exact Sciences, participated in a study that discovered a new type of stellar explosion – an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. Such supernovas arise from the explosions of stars 8-9 times the mass of the sun. The discovery also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was seen by ancient astronomers, before eventually becoming the Crab Nebula, that we know today.

A supernova is the explosion of a star following a sudden imbalance between two opposing forces that shaped the star throughout its life. Gravity tries to contract every star. Our sun, for example, counter balances this force through nuclear fusion in its core, which produces pressure that opposes the gravitational pull. As long as there is enough nuclear fusion, gravity will not be able to collapse the star. However, eventually, nuclear fusion will stop, just like gas runs out in a car, and the star will collapse. For stars like the sun, the collapsed core is called a white dwarf. This material in white dwarfs is so dense that quantum forces between electrons prevent further collapse.

For stars 10 times more massive than our sun, however, electron quantum forces are not enough to stop the gravitational pull, and the core continues to collapse until it becomes a neutron star or a black hole, accompanied by a giant explosion. In the intermediate mass range, the electrons are squeezed (or more accurately, captured) onto atomic nuclei. This removes the electron quantum forces, and causes the star to collapse and then explode.

Historically, there have been two main supernova types. One is a thermonuclear supernova — the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. Another main supernova type is a core-collapse supernova where a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and has its core collapsed, creating a black hole or a neutron star. Theoretical work suggested that electron-capture supernovae would occur on the borderline between these two types of supernovae.

That’s the theory that was developed in the 1980’s by Ken’ichi Nomoto of the University of Tokyo, and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova. The stars should lose a lot of mass of particular composition before exploding, and the supernova itself should be relatively weak, have little radioactive fallout, and produce neutron-rich elements.

The new study, published in Nature Astronomy, focuses on the supernova SN2018zd, discovered in 2018 by Japanese amateur astronomer Koihchi Itagaki. Dr. Iair Arcavi, of the astrophysics department at Tel Aviv University, also took part in the study. This supernova, located in the galaxy NGC 2146, has all of the properties expected from an electron-capture supernova, which were not seen in any other supernova. In addition, because the supernova is relatively nearby – only 31 million light years away – the researchers were able to identify the star in pre-explosion archival images taken by the Hubble Space Telescope. Indeed, the star itself also fits the predictions of the type of star that should explode as an electron-capture supernovae, and is unlike stars that were seen to explode as the other types of supernovae.

While some supernovae discovered in the past had a few of the indicators predicted for electron-capture supernovae, only SN2018zd had all six – a progenitor star that fits within the expected mass range, strong pre-supernova mass loss, an unusual chemical composition, a weak explosion, little radioactivity, and neutron-rich material. “We started by asking ‘what’s this weirdo?'” said Daichi Hiramatsu of the University of California Santa Barbara and Las Cumbres Observatory, who led the study. “Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario.”

The new discoveries also illuminate some mysteries of one of the most famous supernovae of the past. In A.D. 1054 a supernova happened in our own Milky Way Galaxy, and according to Chinese and Japanese records, it was so bright that it could be seen in the daytime and cast shadows at night. The resulting remnant, the Crab Nebula, has been studied in great detail, and was found to have an unusual composition. It was previously the best candidate for an electron-capture supernova, but this was uncertain partly because the explosion happened nearly a thousand years ago. The new result increases the confidence that the historic 1054 supernova was an electron-capture supernova.

“It’s amazing that we can shed light on historical events in the Universe with modern instruments,” says Dr. Arcavi. “Today, with robotic telescopes that scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1000 years between one event and the next.”

Dr. Arcavi is a member of the Global Supernova Project, and makes use of the Las Cumbres telescope network to study rare transient phenomena like supernovae, neutron star mergers, and stars torn apart by black holes.

Link to the original article: https://www.nature.com/articles/s41550-021-01384-2

Featured image: Hubble Space Telescope color composite of the electron-capture supernova 2018zd and the host starburst galaxy NGC 2146 © NASA/STScI/J. DePasquale; Las Cumbres Observatory

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Reference: Hiramatsu, D., Howell, D.A., Van Dyk, S.D. et al. The electron-capture origin of supernova 2018zd. Nat Astron (2021). https://doi.org/10.1038/s41550-021-01384-2

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Provided by Tel-Aviv University

Teardrop Star Reveals Hidden Supernova Doom (Planetary Science)

• International team led by University of Warwick makes rare sighting of a binary star system heading towards supernova.
• Star system’s fate was identified from its unusual light variations, a sign that one star has been distorted into a teardrop shape by a massive white dwarf companion.
• Supernovas from such star systems can be used as ‘standard candles’ to measure expansion of the universe.

Astronomers have made the rare sighting of two stars spiralling to their doom by spotting the tell-tale signs of a teardrop-shaped star.

The tragic shape is caused by a massive nearby white dwarf distorting the star with its intense gravity, which will also be the catalyst for an eventual supernova that will consume both. Found by an international team of astronomers and astrophysicists led by the University of Warwick, it is one of only very small number of star systems that has been discovered that will one day see a white dwarf star reignite its core.

New research published by the team today (12 July) in Nature Astronomy confirms that the two stars are in the early stages of a spiral that will likely end in a Type Ia supernova, a type that helps astronomers determine how fast the universe is expanding.

This research received funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the Science and Technology Facilities Council, part of UK Research and Innovation.

HD265435 is located roughly 1,500 light years away and comprises a hot subdwarf star and a white dwarf star orbiting each other closely at a rate of around 100 minutes. White dwarfs are ‘dead’ stars that have burnt out all their fuel and collapsed in on themselves, making them small but extremely dense.

A type Ia supernova is generally thought to occur when a white dwarf star’s core reignites, leading to a thermonuclear explosion. There are two scenarios where this can happen. In the first, the white dwarf gains enough mass to reach 1.4 times the mass of our Sun, known as the Chandrasekhar limit. HD265435 fits in the second scenario, in which the total mass of a close stellar system of multiple stars is near or above this limit. Only a handful of other star systems have been discovered that will reach this threshold and result in a Type Ia supernova.

Lead author Dr Ingrid Pelisoli from the University of Warwick Department of Physics, and formerly affiliated with the University of Potsdam, explains:

“We don’t know exactly how these supernovae explode, but we know it has to happen because we see it happening elsewhere in the universe.

“One way is if the white dwarf accretes enough mass from the hot subdwarf, so as the two of them are orbiting each other and getting closer, matter will start to escape the hot subdwarf and fall onto the white dwarf. Another way is that because they are losing energy to gravitational wave emissions, they will get closer until they merge. Once the white dwarf gains enough mass from either method, it will go supernova.”

Using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the team were able to observe the hot subdwarf, but not the white dwarf as the hot subdwarf is much brighter. However, that brightness varies over time which suggested the star was being distorted into a teardrop shape by a nearby massive object. Using radial velocity and rotational velocity measurements from the Palomar Observatory and the W. M. Keck Observatory, and by modelling the massive object’s effect on the hot subdwarf, the astronomers could confirm that the hidden white dwarf is as heavy as our Sun, but just slightly smaller than the Earth’s radius.

Combined with the mass of the hot subdwarf, which is a little over 0.6 times the mass of our Sun, both stars have the mass needed to cause a Type Ia supernova. As the two stars are already close enough to begin spiralling closer together, the white dwarf will inevitably go supernova in around 70 million years. Theoretical models produced specifically for this study predict that the hot subdwarf will contract to become a white dwarf star as well before merging with its companion.

Type Ia supernovae are important for cosmology as ‘standard candles’. Their brightness is constant and of a specific type of light, which means astronomers can compare what luminosity they should be with what we observe on Earth, and from that work out how distant they are with a good degree of accuracy. By observing supernovae in distant galaxies, astronomers combine what they know of how fast this galaxy is moving with our distance from the supernova and calculate the expansion of the universe.

Dr Pelisoli adds:

“The more we understand how supernovae work, the better we can calibrate our standard candles. This is very important at the moment because there’s a discrepancy between what we get from this kind of standard candle, and what we get through other methods.

“The more we understand about how supernovae form, the better we can understand whether this discrepancy we are seeing is because of new physics that we’re unaware of and not taking into account, or simply because we’re underestimating the uncertainties in those distances.

“There is another discrepancy between the estimated and observed galactic supernovae rate, and the number of progenitors we see. We can estimate how many supernovae are going to be in our galaxy through observing many galaxies, or through what we know from stellar evolution, and this number is consistent. But if we look for objects that can become supernovae, we don’t have enough. This discovery was very useful to put an estimate of what a hot subdwarf and white dwarf binaries can contribute. It still doesn’t seem to be a lot, none of the channels we observed seems to be enough.”

Featured image: Artist’s impression of the HD265435 system at around 30 million years from now, with the smaller white dwarf distorting the hot subdwarf into a distinct ‘teardrop’ shape. Credit: University of Warwick/Mark Garlick

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Reference

• ‘A hot subdwarf–white dwarf super-Chandrasekhar candidate supernova Ia progenitor ’ will be published in Nature Astronomy, DOI: 10.1038/s41550-021-01413-0 Link: https://www.nature.com/articles/s41550-021-01413-0 (to go live after embargo)

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Provided by University of Warwick

New Type Of Massive Explosion Explains Mystery Star (Planetary Science)

‘Magneto-rotational hypernova’ soon after the Big Bang fuelled high levels of uranium, zinc in ancient stellar oddity

A massive explosion from a previously unknown source – 10 times more energetic than a supernova – could be the answer to a 13-billion-year-old Milky Way mystery.

Astronomers led by David Yong, Gary Da Costa and Chiaki Kobayashi from Australia’s ARC Centre of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D) based at the Australian National University (ANU) have potentially discovered the first evidence of the destruction of a collapsed rapidly spinning star – a phenomenon they describe as a “magneto-rotational hypernova”.

The previously unknown type of cataclysm – which occurred barely a billion years after the Big Bang – is the most likely explanation for the presence of unusually high amounts of some elements detected in another extremely ancient and “primitive” Milky Way star.

That star, known as SMSS J200322.54-114203.3, contains larger amounts of metal elements, including zinc, uranium, europium and possibly gold, than others of the same age.

Neutron star mergers – the accepted sources of the material needed to forge them – are not enough to explain their presence.

The astronomers calculate that only the violent collapse of a very early star – amplified by rapid rotation and the presence of a strong magnetic field – can account for the additional neutrons required.

The research is published today in the journal Nature.

“The star we’re looking at has an iron-to-hydrogen ratio about 3000 times lower than the Sun – which means it is a very rare: what we call an extremely metal-poor star,” said Dr Yong, who is based at the ANU.

“However, the fact that it contains much larger than expected amounts of some heavier elements means that it is even rarer – a real needle in a haystack.”

The first stars in the universe were made almost entirely of hydrogen and helium. At length, they collapsed and exploded, turning into neutron stars or black holes, producing heavier elements which became incorporated in tiny amounts into the next generation of stars – the oldest still in existence.

Rates and energies of these star deaths have become well known in recent years, so the amount of heavy elements they produce is well calculated. And, for SMSS J200322.54-114203.3, the sums just don’t add up.

“The extra amounts of these elements had to come from somewhere,” said Associate Professor Chiaki Kobayashi from the University of Hertfordshire, UK.

“We now find the observational evidence for the first time directly indicating that there was a different kind of hypernova producing all stable elements in the periodic table at once — a core-collapse explosion of a fast-spinning strongly-magnetized massive star. It is the only thing that explains the results.”

Hypernovae have been known since the late 1990s. However, this is the first time one combining both rapid rotation and strong magnetism has been detected.

“It’s an explosive death for the star,” said Dr Yong. “We calculate that 13 billion-years ago J200322.54-114203.3 formed out of a chemical soup that contained the remains of this type of hypernova. No one’s ever found this phenomenon before.”

J200322.54-114203.3 lies 7500 light-years from the Sun, and orbits in the halo of the Milky Way.

Another co-author, Nobel Laureate and ANU Vice-Chancellor Professor Brian Schmidt, added, “The high zinc abundance is definite marker of a hypernova, a very energetic supernova.”

Head of the First Stars team in ASTRO 3D, Professor Gary Da Costa from ANU, explained that the star was first identified by a project called the SkyMapper survey of the southern sky.

“The star was first identified as extremely metal-poor using SkyMapper and the ANU 2.3m telescope at Siding Spring Observatory in western NSW,” he said. “Detailed observations were then obtained with the European Southern Observatory 8m Very Large Telescope in Chile.”

ASTRO 3D director, Professor Lisa Kewley, commented: “This is an extremely important discovery that reveals a new pathway for the formation of heavy elements in the infant universe.”

Other members of the research team are based at the Massachusetts Institute of Technology in the US, Stockholm University in Sweden, the Max Planck Institute for Astrophysics in Germany, Italy’s Istituto Nazionale di Astrofisica, and Australia’s University of New South Wales.

Featured image: The star SMSS J200322.54-114203.3. (centre, with crosshairs) in the south-eastern corner of the constellation Aquila (the Eagle) close to the border with Capricornus and Sagittarius. Credit: Da Costa/SkyMapper

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Reference: Yong, D., Kobayashi, C., Da Costa, G.S. et al. r-Process elements from magnetorotational hypernovae. Nature 595, 223–226 (2021). https://doi.org/10.1038/s41586-021-03611-2

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Provided by ASTRO3D

Are There Any Structural Changes In Deeper Layers Of The Sun? (Planetary Science)

Sarbani Das in her recent paper examined whether or not there are any changes in the deeper layers of the sun by using helioseismic data for two solar cycles: solar cycle 23 and 24. She found that there are significant changes in the solar convection zone and that the sound speed in the solar convection zone decreases compared to sound speed below it as the sun becomes more active. Findings of her study recently appeared in Arxiv.

The solar interior is divided into four regions by the different processes that occur there. The innermost region is the “core”, where energy is generated. This energy diffuses outward by radiation through the radiative zone and by convective fluid which flows through the convection zone. The thin interface layer called the “tachocline”, present between the radiative zone and the convection zone is where the Sun’s magnetic field is thought to be generated. Several studies demonstrated that, there are changes in solar dynamics and it is quite easy to detect. However, no studies demonstrated changes in solar structure, as it is very hard to detect. Thus, this inspired Sarbani Das to examined whether there are any changes in the deeper layers of the sun or not.

By using helioseismic data obtained over two solar cycles, and sacrificing resolution in favour of lower uncertainties, she showed that there are significant changes in the solar convection zone.

“Using MDI data, we find a relative squared sound-speed difference of 2.56×10¯⁵ at the convection-zone base between the maximum of solar Cycle~23 and the minimum between Cycles~23 and 24. The squared sound-speed difference for the maximum of Cycle~24 obtained with HMI data is 1.95×10¯⁵. GONG data support these results.”

— Sarbani Das, Astrophysicist and Professor in the Department of Astronomy, Yale University

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She also found that, as solar activity increases, the sound speed in the region above the base of the convection zone, i.e., the tachocline region, decreases compared with that below the convection-zone base. This result is consistent with the assumption that the sound-speed changes are a result of magnetic fields.

Inversion results for the difference in sound speed below and above the convection-zone base. © Sarbani Das

Moreover, she showed that there is a change in the gradient with the solar cycle. This change is most significant at base of the convection zone. This implies a change in the position of the convection zone base. However, she couldn’t able to confirm this change with current results or available frequencies.

“The results are intriguing enough to examine the results further. Given that the current data sets are not ideal, the analyses need to be repeated with frequencies obtained from time series that are longer; while longer time series will smooth out some of the solar-cycle related changes, overlapping sets will still reveal the changes. Two-year data sets at the solar maxima and two years at the minima and a few two-year sets in between will help.”

— she concluded

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Reference: Sarbani Das, “Evidence of solar-cycle related structural changes in the solar convection zone”, Arxiv, pp. 1-11, 2021. arXiv:2106.08383

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